This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2000-202340, filed Jul. 4, 2000, the entire contents of which are incorporated herein by reference.
1. Field of the Invention
The present invention relates to a color cathode ray tube, and more particularly, to a color cathode ray tube having the outer surface of an effective portion of its panel flattened.
2. Description of the Related Art
In order to display a color image without a color drift on a phosphor screen of a color cathode ray tube, in general, three electron beams that are passed through electron beam holes of a shadow mask must be landed correctly on their corresponding three-color phosphor layers of the screen. To attain this, the shadow mask should be accurately located in a given position relative to a panel. It is necessary, therefore, to set the distance (value q) between the panel and the shadow mask accurately and appropriately.
If the pitch of the three-color phosphor layers, that is, the interval between each two adjacent phosphor layers of each color, out of the phosphor layers of three colors arranged in a given order (e.g., in the order of red (R), green (G), blue (B), red (R), . . .), is PHp, it is ideal to adjust an interval d between two of each three adjacent phosphor layers to d=(⅔) PHp, in order to set the value q appropriately. If the value q is not set appropriately for the phosphor layer pitch PHp, however, black non-luminous layers cannot be wide enough, so that the color purity easily lowers as the color image is displayed. The black non-luminous layers can be assured of a satisfactory width if the phosphor layer pitch PHp is wide enough. If the phosphor layer pitch PHp is too wide, however, the resolution will be lowered.
In order to improve the visibility of modern color cathode ray tubes, moreover, the curvature of the outer surface of the panel is expected to be reduced (or the radius of curvature be increased) so that the outer surface is substantially flat. For higher visibility, therefore, the curvature of the inner surface of the panel must be also reduced. In order to land the electron beams accurately on the phosphor layers of the inner surface of the panel, moreover, the value q must be set appropriately, as mentioned before, so that the curvature of the body of the shadow mask that has the electron beam holes must be also reduced to match the inner surface of the panel.
If the curvature of the shadow mask body is reduced, however, the mechanical strength of the shadow mask lowers, so that the mask may be deformed in manufacturing processes for a cathode ray tube. If the curvature of the shadow mask body is reduced, moreover, the shadow mask howls against voices or sounds from a TV set in which the color cathode ray tube is incorporated. The deformation or howling of the shadow mask causes dislocation of beam landing. If the electron beams traverse the black non-luminous layers and cause any other phosphor layers than the phosphor layer of a desired color to glow, owing to the dislocation of beam landing, the color purity is lowered.
According to the principle of operation of the color cathode ray tube, the electron beams that pass through the electron beam holes of the shadow mask and reach the phosphor screen account for ⅓ or less of all the electron beams that are emitted from an electron gun structure. The other electron beams having failed to reach the phosphor screen run against any other portions of the shadow mask than the electron beam holes and are converted into thermal energy, whereby the shadow mask is heated. The resulting thermal expansion causes so-called doming such that the shadow mask bulges toward the phosphor screen. If the distance between the phosphor screen and the shadow mask, that is, the value q, exceeds its tolerance limit, beam landing on the phosphor layers is dislocated. Thus, the electron beams traverse the black non-luminous layers and cause some other phosphor layers than the phosphor layer of the desired color to glow, thereby lowering the color purity.
The dislocation of beam landing that is attributable to the thermal expansion of the shadow mask substantially varies depending on the brightness of displayed image patterns, the duration of the patterns, etc. If a high-brightness image pattern is displayed locally, in particular, local doming occurs, so that local dislocation of beam landing is caused in a short time.
The beam landing dislocation that is attributable to the local doming occurs most prominently when the high-brightness image pattern is displayed in a region at a distance corresponding to about ⅓ of the distance between a pair of short sides (or the overall width in the direction of the major axis) of the screen from the center of the screen. Accordingly, the black non-luminous layers should be made as wide as possible in order to prevent landing mistake in this region.
Thus, intervals between electron beam hole rows in the shadow mask body are simply increased from the center of the screen toward the periphery to maintain the width of the black non-luminous layers in the peripheral portion of the screen without lowering the mechanical strength of the shadow mask body. In this case, however, it is hard to maintain the flatness of the panel without lowering the mechanical strength of the shadow mask and fully to prevent deterioration of color purity that is attributable to local doming.
The present invention has been contrived in consideration of these circumstances, and its object is to provide a color cathode ray tube with high visibility, in which deterioration of color purity attributable to deformation or doming of a shadow mask is lessened.
According to the present invention, there is provided a color cathode ray tube comprising: an envelope including a panel, having a substantially rectangular effective portion with a substantially flat outer surface, and a funnel bonded to the panel; a phosphor screen formed on an inner surface of the panel and including phosphor layers and black non-luminous layers; an electron gun structure located in a neck of the funnel and capable of emitting electron beams toward the phosphor screen; and a shadow mask opposed to the phosphor screen and including a substantially rectangular mask body formed having a large number of electron beam holes and a mask frame supporting of the mask body, the envelope having a tube axis extending through the respective centers of the effective portion and the electron gun structure, a major axis extending at right angles to the tube axis, and a minor axis extending at right angles to the tube axis and the major axis, the interval between each two adjacent electron beam holes on the major axis of the mask body being greater at the major axis end of the mask body than in the center, the interval between the electron beam holes increasing toward the major axis end so that the rate of change of the interval has a relative maximum value in the region at the distance of L/4 to 3L/4 from the center of the mask body, where L is the distance from the center of the mask body to the major axis end along the major axis.
Even if the curvature of the outer surface of the effective portion of the panel is reduced to improve visibility, according to the color cathode ray tube constructed in this manner, the color purity can be prevented from being lowered by dislocation of beam landing that is attributable to local doming of the shadow mask body or deformation of the shadow mask body caused in a manufacturing process or by external impact, so that improved image quality can be ensured.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.